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Abstract:

A neutron detector with monolithically integrated readout circuitry,
including: a bonded semiconductor die; an ion chamber formed in the
bonded semiconductor die; a first electrode and a second electrode formed
in the ion chamber; a neutron absorbing material filling the ion chamber;
and the readout circuitry which is electrically coupled to the first and
second electrodes. The bonded semiconductor die includes an etched
semiconductor substrate bonded to an active semiconductor substrate. The
readout circuitry is formed in a portion of the active semiconductor
substrate. The ion chamber has a substantially planar first surface on
which the first electrode is formed and a substantially planar second
surface, parallel to the first surface, on which the second electrode is
formed. Desirably, the distance between the first electrode and the
second electrode may be equal to or less than the 50% attenuation length
for neutrons in the neutron absorbing material filling the ion chamber.

Claims:

1. comprising: a thermal neutron detector; a high pressure ion chamber
formed in a dielectric material, the high pressure ion chamber having a
substantially planar first surface and a substantially planar second
surface parallel to the first surface; a first electrode formed on the
first surface of the high pressure ion chamber; a second electrode formed
on the second surface of the high pressure ion chamber, wherein
electronics are embedded within a device layer of a wafer; a neutron
absorbing material filling the high pressure ion chamber, a chamber
pressure of the neutron absorbing material being equal to or greater than
100 atm, wherein the high pressure ion chamber increases utilization of
the neutron absorbing material due to reduced self-absorption and uniform
distribution of absorbing material as enabled by the substantially planar
first surface and the substantially planar second surface parallel to the
first surface; and a neutron moderating material encompassed by the
thermal neutron detector.

2. (canceled)

3. The system claim 1, wherein a cross-section of the high pressure ion
chamber parallel to the first electrode is one of rectangular or round.

4. (canceled)

5. The system of claim 1, wherein the dielectric material is one of
silicon or FR4 glass laminate.

6. The system of claim 1, wherein the neutron absorbing material includes
at least one of: helium-3; helium-4; xenon; hydrogen; propane; or
methane.

7. The system of claim 1, wherein the chamber pressure of the neutron
absorbing material is sufficient to liquefy the neutron absorbing
material.

8. The system of claim 1, wherein structure of the neutron moderating
material encompassed by the thermal neutron detector improves efficiency
of the thermal neutron detector and minimizes total weight and volume of
the system as compared to the thermal neutron detector embedded in the
neutron moderating material.

9. The system of claim 1, wherein the neutron moderating material is
polyethylene.

10. A neutron detector with monolithically integrated readout circuitry,
comprising: a bonded semiconductor die including an etched semiconductor
substrate bonded to an active semiconductor substrate; an ion chamber
formed in the bonded semiconductor die, the ion chamber having a
substantially planar first surface and a substantially planar second
surface parallel to the first surface; a first electrode formed on the
first surface of the ion chamber; a second electrode formed on the second
surface of the ion chamber; a neutron absorbing material filling the ion
chamber; and the readout circuitry formed in a portion of the active
semiconductor substrate and electrically coupled to the first electrode
and the second electrode

11. The neutron detector of claim 10, wherein a distance between the
first electrode and the second electrode is equal to or less than a 50%
attenuation length for neutrons in the neutron absorbing material filling
the ion chamber.

12. The neutron detector of claim 10, wherein a cross-section of the ion
chamber parallel to the first electrode is one of rectangular or round.

13. The neutron detector of claim 12, wherein an area of the
cross-section of the ion chamber parallel to the first electrode is
greater than or equal to 100 times a distance between the first electrode
and the second electrode squared.

14. The neutron detector of claim 10, wherein the neutron absorbing
material includes at least one of: helium-3; helium-4; xenon; hydrogen;
propane; or methane; or a hydrogenous material.

15. The neutron detector of claim 10, wherein a chamber pressure of the
neutron absorbing material filling the ion chamber is equal to or greater
than 100 atm.

16. The neutron detector of claim 15, wherein the chamber pressure of the
neutron absorbing material is sufficient to liquefy the neutron absorbing
material.

18. The neutron detector of claim 10, wherein: the active semiconductor
substrate of the bonded semiconductor die is a silicon on insulator (SOI)
substrate; and the readout circuitry includes radiation hardened
electronics formed using an SOI radiation hardened process.

20. A directional neutron detector, comprising: an ion chamber formed in
a dielectric material, the ion chamber having a substantially planar
first surface and a substantially planar second surface parallel to the
first surface and located a predetermined distance from the first surface
along a normal to the first surface; a signal electrode formed on the
first surface of the ion chamber; a ground electrode formed on the second
surface of the ion chamber; a neutron recoil/scattering material filling
the ion chamber, a chamber pressure of the neutron recoil/scattering
material being selected such that a reaction particle ion trail length
for neutrons absorbed by the neutron recoil/scattering material is equal
to or less than the predetermined distance between the first surface and
the second surface of the ion chamber; readout circuitry electrically
coupled to the signal electrode and the ground electrode, and adapted to
generate a time varying signal proportional to charge collected by the
signal electrode as a function of time, the collected charge originating
from absorption of neutrons by the neutron recoil/scattering material,
the time varying signal including a pulse corresponding to each absorbed
neutron; and a signal processor electrically coupled to the readout
circuitry and adapted to determine a path angle relative to the normal to
the first surface of the ion chamber for each absorbed neutron based on a
rise time of the corresponding pulse in the time varying signal.

21. The directional neutron detector of claim 20, wherein the reacting
particle ranges and capillary dimensions are controlled to provide an
inherent directionality of the neutron detector.

22. The directional neutron detector of claim 20, wherein a cross-section
of the ion chamber parallel to the first surface is one of rectangular or
round.

23. The directional neutron detector of claim 22, wherein the reacting
particle's range is smaller than at least one of the dimensions x, y, z
of the volume of the neutron recoil/scattering material containing the
reacting particle.

24. The directional neutron detector of claim 20, wherein the dielectric
material is one of silicon or FR4.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part application and claims
the priority benefit of U.S. application Ser. No. 12/046,041, filed Mar.
11, 2008 and entitled "GAS-FILLED MICROCHANNEL ARRAY NEUTRON DETECTOR."
The disclosure of this application is incorporated herein by reference.

TECHNICAL FIELD

[0003] The present invention is directed generally to neutron detectors,
and, more particularly, to neutron detectors based on ion chambers.

BACKGROUND OF THE INVENTION

[0004] There are two underlying issue that motivated the disclosed
inventions. The first was to improve capabilities for finding special
nuclear materials (SNM). The second was to develop potential approaches
to improve the capabilities for characterizing or assessing SNM. To get
the most information from a radiation field for both of those goals, the
ideal sensor would need to discriminate against all forms of background
as well as measure energy spectra and image the location of the
radiation. The present invention represents an attempt to develop this
ideal sensor system and to meet these goals concurrently.

[0005] Cost is an issue however the need is great enough and value
acceptable. The cost of this type of unique development is likely to be
much less than that which has already gone into helium-3 replacement
technologies to date and is currently is being spent on scintillator
improvements. The technology also offers functionality not provided by
current alternatives.

[0006] Current neutron detection technology is primarily focused on bulk
thermal neutron detectors such as commercial helium-3 tubes or fast
neutron detection utilizing liquid or plastic scintillators. Helium-3
detectors are less desirable because of the limited supply of helium-3.
Additionally, current helium-3 detectors are bulky and, aside from
gaseous electron multipliers (GEMs), the helium-3 proportional tubes do
not routinely generate two dimensional information. Multiwire helium-3
proportional detectors have been investigated, but these use a lot of the
helium-3 gas and can be difficult to field.

[0007] Liquid scintillators have disadvantages in that they are gamma
sensitive, the liquids are often flammable and they require bulky
photomultiplier tubes. They can discriminate between neutrons and gammas
at up to 10 MHz in a laboratory environment, but in intense short-pulsed
situations, such as that encountered in active detection, pulse shape
discrimination may not be fast enough to acquire the data and resolve
gamma background from neutron signals. Liquid scintillators (many of
which do not provide pulse shape discrimination against gammas) can be
made pixelated, but offer no capability for spectral or angular
discrimination without the need for a heavy shield or aperture system.

[0008] Exemplary embodiments of the present invention may improve gamma
rejection and reduce the consumption of helium-3, as well as providing
for increased resolution and directional discrimination. These and other
advantages of the present invention may be understood by those skilled in
the art from the following detailed description.

SUMMARY OF THE INVENTION

[0009] An exemplary embodiment of the present invention is a thermal
neutron detector, including: a high pressure ion chamber formed in a
dielectric material; first and second electrodes formed in the high
pressure ion chamber; a neutron absorbing material filling the ion
chamber; and a neutron moderating material surrounding at least a portion
of the high pressure ion chamber. The high pressure ion chamber has a
substantially planar first surface on which the first electrode is formed
and a substantially planar second surface, parallel to the first surface,
on which the second electrode is formed. The chamber pressure of the
neutron absorbing material is equal to or greater than 100 atm.

[0010] Another exemplary embodiment of the present invention is a neutron
detector with monolithically integrated readout circuitry, including: a
bonded semiconductor die; an ion chamber formed in the bonded
semiconductor die; a first electrode and a second electrode formed in the
ion chamber; a neutron absorbing material filling the ion chamber; and
the readout circuitry which is electrically coupled to the first and
second electrodes. The bonded semiconductor die includes an etched
semiconductor substrate bonded to an active semiconductor substrate. The
readout circuitry is formed in a portion of the active semiconductor
substrate. The ion chamber has a substantially planar first surface on
which the first electrode is formed and a substantially planar second
surface, parallel to the first surface, on which the second electrode is
formed.

[0011] A further exemplary embodiment of the present invention is a
directional neutron detector, including: an ion chamber formed in a
dielectric material; a signal electrode and aground electrode formed in
the ion chamber; a neutron absorbing material filling the ion chamber;
readout circuitry which is electrically coupled to the signal and ground
electrodes; and a signal processor electrically coupled to the readout
circuitry. The ion chamber has a substantially planar first surface on
which the signal electrode is formed and a substantially planar second
surface, parallel to the first surface, on which the ground electrode is
formed. The second surface is located a predetermined distance from the
first surface along the normal to the first surface. The chamber pressure
of the neutron absorbing material is selected such that the reaction
particle ion trail length for neutrons absorbed by the neutron absorbing
material is equal to or less than the predetermined distance between the
first surface and the second surface of the ion chamber. The readout
circuitry is adapted to generate a time varying signal proportional to
the charge collected by the signal electrode as a function of time. The
collected charge originates from absorption of neutrons by the neutron
absorbing material, and the time varying signal includes a pulse
corresponding to each absorbed neutron. The signal processor is adapted
to determine a path angle relative to the normal to the first surface of
the ion chamber for each absorbed neutron based on the rise time of the
corresponding pulse in the time varying signal.

[0012] It is to be understood that both the foregoing general description
and the following detailed description are exemplary, but are not
restrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The invention is best understood from the following detailed
description when read in connection with the accompanying drawing. It is
emphasized that, according to common practice, the various features of
the drawing are not to scale. On the contrary, the dimensions of the
various features are arbitrarily expanded or reduced for clarity.
Included in the drawing are the following figures:

[0015] FIG. 2A is a side cut-away drawing of an exemplary ion chamber
based thermal neutron detector according to the present invention;

[0016]FIG. 2B is a top plan drawing of the exemplary ion chamber based
thermal neutron detector of FIG. 2A;

[0017] FIG. 3A is a side cut-away drawing of an exemplary ion chamber
based neutron detector with monolithically integrated readout circuitry
according to the present invention;

[0018] FIG. 3B is a top plan drawing of the exemplary ion chamber based
neutron detector of FIG. 3A;

[0019]FIG. 4 is a schematic drawing of an exemplary directional neutron
detector according to the present invention;

[0020] FIG. 5A is a side cut-away drawing of an alternative exemplary ion
chamber based neutron detector according to the present invention;

[0021]FIG. 5B is a top plan drawing of the alternative exemplary ion
chamber based neutron detector of FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Exemplary embodiments of the present invention include a number of
designs for ion chamber based neutron detectors that may be used for
thermal and/or fast neutron detection. These exemplary ion chamber based
neutron detectors may include monolithically integrated readout
circuitry. Additionally, these exemplary ion chamber neutron detectors
may be designed to detect the direction of flight of detected neutrons,
thereby allowing for faster and simpler localization and identification
of the neutron source.

[0024] There is considerable room for improvement in system efficiency as
well as in making more efficient use of helium-3 gas in a neutron
detector of this type. Prior art cylindrical proportional tube neutron
detection systems, such as prior art proportion tube neutron detector
100, typically have relatively poor utilization of the helium-3 for
thermal neutron detection. This utilization may be somewhat higher for
fast neutron, but is often still less than desirable. The low utilization
rate arises in part because of the cylindrical geometry of the
proportional tubes. Neutrons do not penetrate to the center of the tubes
and therefore do not uniformly sample the gas in the system. The effect
of moderating materials used in thermal neutron detectors is non-uniform
as well. A preferable system would have helium-3 uniformly dispersed
across the face of a detector and be uniformly moderated.

[0025] Experiments, using a slab geometry at three different pressures,
which have conducted by the inventors show that the attenuation of
neutrons in helium-3 gas is a function of depth and pressures. One
notable result of these experiments is that for a 10 atm pressure, the
attenuation of thermal neutrons (0.025 eV) is about 50% at 5 mm. However,
the tube diameter of a typical prior art proportion tube neutron detector
is on the order of 2.cm. This means that most of the thermal neutrons do
not penetrate to the core of the tube and those atoms away from the edge
of the tube do not see the neutron flux. In other words, because the gas
self-shields and removes neutrons, the gas inside the tube does not
sample the neutron flux as effectively as the gas at the edge. Because of
this self-shielding, the most effective use of the helium-3 atoms along
neutron path 108 occurs in uniform sheet 106. Another key implication is
that as the total absorption (intrinsic efficiency) increases, the
efficiency per atom decreases. This is because there are fewer neutrons
to interact with each additional helium-3 atom. Together, these features
imply that outer uniform sheet 106 of neutron absorber (often helium-3)
in prior art proportion tube neutron detector 100 shield, or self-shield,
the core neutron absorber from the neutrons one wishes to detect. This
effect may be more pronounced for thermal neutrons than for fast neutron,
which may penetrate further into the neutron absorber, thus increasing
the width of uniform sheet 106.

[0026] FIGS. 2A and 2B illustrate exemplary ion chamber based thermal
neutron detector 200 according to the present invention. Exemplary ion
chamber based thermal neutron detector 200 includes: high pressure ion
chamber 202, which is filled with neutron absorbing material 204; bottom
electrode 206 formed on the bottom surface of high pressure ion chamber
202; top electrode 208 formed on the top surface of the high pressure ion
chamber 202; and neutron moderating material 210, which surround at least
a portion of high pressure ion chamber 202. Reference herein to the top
and bottom sides of exemplary ion chamber based thermal neutron detector
200 are used as illustrated in the side cut away drawing of FIG. 2A.
These references are merely for clarity and are not intended to be
limiting.

[0027] It is noted that although neutron moderating material 210 is
illustrated in FIG. 2A as surrounding only the top portion of high
pressure ion chamber 202, in other exemplary embodiments it may surround
both the top and bottom portions of high pressure ion chamber 202, or
high pressure ion chamber 202 may be completely surrounded by neutron
moderating material 210. Neutron moderating material 210 may be any
material known to moderate neutrons, one example being polyethylene.

[0028] High pressure ion chamber 202 is desirably formed of a dielectric
material, such as silicon or FR4 glass laminate. This allows high
pressure ion chamber 202 to be filled with neutron absorbing material 204
at pressures in excess of 100 atm, possibly exceeding 10,000 atm. At the
highest chamber pressures, neutron absorbing material 204 may even become
liquefied. (For particular thin high pressure ion chambers 202, surface
or capillary effects may play a role in liquefaction of neutron absorbing
material 204.)

[0029] The bottom and top surfaces of high pressure ion chamber 202 (on
which bottom electrode 206 and top electrode 208 are formed,
respectively) are desirably substantially planar and parallel to one
another. Bottom electrode 206 and top electrode 208 may formed by
depositing a metal or other conductor on these surfaces; or in the case
in which high pressure ion chamber 202 is formed from silicon (or another
material with semiconducting properties), bottom electrode 206 and top
electrode 208 may be formed by doping these surfaces to provide
sufficient conductivity.

[0030] Neutron absorbing material 204 may be one of a number of materials
such as: helium-3; helium-4; xenon; hydrogen; propane; or methane;
however, a combination of helium-3 and xenon may be preferable. One issue
with this choice of material is the limited supply of helium-3.
Therefore, more efficient use of neutron absorbing material 204 is very
desirable. As discussed above, thermal neutron flux is rapidly attenuated
in helium-3 even at a modest pressure of 10 atm. This attenuation is even
more rapid at higher pressures. Thus, to improve the efficiency, per
atom, of neutron absorbing material 204, distance 106' between bottom
electrode 206 and top electrode 208 is desirably equal to or less than
the 50% attenuation length for thermal neutrons in neutron absorbing
material 204 at the chamber pressure. One can think of this as the
process of unrolling the outer uniform sheet 106 of tube 102, where most
of the thermal neutron interactions occurred in prior art proportion tube
neutron detector 100. The area of the cross-section of high pressure ion
chamber 202 parallel to electrodes 206, 208 is desirably greater than or
equal to 100 times distance 106 squared. This significant increases the
efficiency of the volume of neutron absorbing material 204.

[0031] Another advantage of reducing distance 106 between electrodes 206,
208 is that is reduces recombination of charge before it can be collected
at electrodes 206, 208 by reducing the drift time before collection for
these charges. This reduction in charge drift time may increase
sensitivity of ion chamber based thermal neutron detector 200, allowing
for improved energy resolution, as well as improving resolution for the
rise time of the pulse resulting from a detection event. As described
below with reference to FIG. 4, improved pulse rise time resolution may
be desirable for determining the angle of the neutron path relative to
the normal of the top and bottom surfaces of ion chamber based thermal
neutron detector 200.

[0032] The cross-section of high pressure ion chamber 202 parallel to the
bottom electrode may be round as illustrated in FIG. 2B or it may be
rectangular as illustrated in FIG. 3B. FIGS. 5A and 5B illustrate
alternative ion chamber design 500 that may be employed. In this
alternative design, ion chamber 500 is formed by bonding etched die
portion 502 to flat die portion 504 at bond points 506. Etched die
portion 502 is etched to include an array of columns. This alternative
design may allow for an ion chamber with a larger cross-sectional area
that is able to withstand extremely high pressures.

[0033] FIGS. 3A and 3B illustrate another exemplary embodiment of the
present invention. Exemplary ion chamber based neutron detector 300
includes an ion chamber formed in a bonded semiconductor die and
monolithically integrated readout circuitry 312. The bonded semiconductor
die is formed by bonding etched semiconductor substrate 302 to active
semiconductor substrate 304 at bond points 306. Bonded semiconductor die
may desirable be formed silicon or silicon based materials. In one
exemplary embodiment, etched semiconductor substrate 302 is formed of
silicon and active semiconductor substrate 304 is a silicon on insulator
(SOI) substrate.

[0034] Monolithic integration of the read out circuitry may desirably
reduce the capacitance and, thus, noise associated with read out of the
charge collected from the reaction particle ion trails of detected
neutrons. Combined with increased charge collection (due to reduced
recombination) and improved pulse shape resolution (due to reduce drift
time) from a reduced distance 106 between electrode 308, 310, the use
monolithically integrated readout circuitry 312 may allow for
significantly improved signal to noise ratio and pulse rise time
determination for exemplary ion chamber based neutron detector 300.
Another advantage of the use of monolithically integrated readout
circuitry 312 may be simplification of designing an exemplary pixelated
ion chamber based neutron detector (not shown) using a two (or three)
dimensional array of individual neutron detectors, such as exemplary ion
chamber based neutron detector 300.

[0035] A hollow space which will form the volume of the ion chamber is
etched into etched semiconductor substrate 302. Top electrode 308 is
formed on the top surface of this hollow space. As noted above with
reference to FIGS. 2A and 2B, reference to the top and bottom herein are
used as illustrated in the side cut away drawing of FIG. 3A; and these
references are merely for clarity and are not intended to be limiting.

[0036] Monolithically integrated readout circuitry 312 is formed in active
semiconductor substrate 304. This circuitry may be formed using a CMOS or
other standard semiconductor fabrication process; however, it may be
desirable to use radiation hardened circuit designs and fabrication
processes. For example, if active semiconductor substrate 304 is an SOI
substrate, monolithically integrated readout circuitry 312 may be
fabricated using an SOI radiation hardened fabrication process.

[0037] Bottom electrode 310 is formed on the opposite surface of active
semiconductor substrate 304, which is desirably parallel to the top
surface of the hollow space of etched semiconductor substrate 302 after
bonding. Although bottom electrode 310 may be electrically coupled to
monolithically integrated readout circuitry 312 later, it may desirable
to electrically couple these elements during the fabrication process.
Monolithically integrated readout circuitry 312 is also electrically
coupled to top electrode 308.

[0038] Fabrication of monolithically integrated readout circuitry 312 and
bottom electrode 310 may desirable be performed before etched
semiconductor substrate 302 is bonded to active semiconductor substrate
304; or alternatively, fabrication of monolithically integrated readout
circuitry 312 and bottom electrode 310 may occur after the bonded
semiconductor die has been formed.

[0040]FIG. 4 illustrates schematically an exemplary directional neutron
detector design according to the present invention. The features of this
exemplary directional neutron detector design may be implemented with any
of the exemplary ion chamber based neutron detectors disclosed above with
reference to FIGS. 2A, 2B, 3A, 3B, 5A, and 5B.

[0041] A neutron following path 108 collides with an atom of neutron
absorbing material 204 creating reaction particle ion trail 110. An
electric field between top electrode 400 and bottom electrode 402 causes
oppositely charged particle to accelerate toward the opposite electrodes.
In the exemplary schematic drawing of FIG. 4, electrons are accelerated
toward top electrode 400 and positively charged ions (not shown) are
accelerated toward bottom electrode 402. The illustrated polarity in
which the neutron enters the ion chamber through a positively charged top
electrode 400 (i.e. the signal electrode) may have the advantage of a
somewhat shorter average drift distances for electrons which may lead to
improved charge collection. (The much slower moving ions may often
recombine with electrons emitted from bottom, or ground, electrode 402
rather than drifting all of the way to this electrode. Therefore, charge
collection is dominated by electron collection at top, or signal,
electrode 400.)

[0042] Charges, electrons and ions, are formed all along reaction particle
ion trail 110. Electrons at the beginning of reaction particle ion trail
110 have a shorter distance 406 to drift before collection and those at
the end of reaction particle ion trail 110 have a longer distance 408 to
drift before collection. A longer distance means a longer time between
formation and collection of these charges. Thus, the rise time of the
pulse of electrons reaching top electrode 400 is dependent on the length
and angle of reaction particle ion trail 110 within the ion chamber. The
length of reaction particle ion trail 110 is dependent on a number of
factors including: the composition and pressure of neutron absorbing
material 204; the energy of the detected neutron; and whether trail is
truncated by one of the sides of the ion chamber. The composition and
pressure of neutron absorbing material 204 may be predetermined. The
energy of the neutron may be determined from the total charge collected
(assuming minimal recombination) and knowledge of the composition and
pressure of neutron absorbing material 204. And the ion chamber size may
be chosen to reduce the probability that the reaction particle ion trail
110 is truncated by a side of the ion chamber. Desirably, a minimum width
of a cross section of the ion chamber parallel to electrodes 400 and 402
is several orders of magnitude larger than the reaction particle ion
trail length for neutrons absorbed by neutron absorbing material 204 at
the chamber pressure. Therefore, for arbitrary incident angles with the
ion chamber only the top and bottom sides have any significant
probability of truncating a reaction particle ion trail. If the distance
between top electrode 400 and bottom electrode 402 is greater than the
typical reaction particle ion trail length for neutrons absorbed by
neutron absorbing material 204 at the chamber pressure the effect of
truncation on the top and bottom sides of the ion chamber may be
desirably low.

[0043] Thus, the length of reaction particle ion trail 110 as a function
of neutron energy may be determined based on predetermined factors with
relatively high certainty. Once the length of the reaction particle ion
trail 110 is determined, the angle of reaction particle ion trail
relative to normal 410 of top electrode 400. As illustrated in FIG. 4,
reaction particle ion trail 110 is approximately collinear to neutron
path 108 for a given detected neutron. Therefore, path angle 412 of
neutron path 108 relative to normal 410 to top electrode 400 may be
calculated from the pulse shape of the collected charge for reaction
particle ion trail 110.

[0044] Electrodes 400 and 402 are electrically coupled to processor
circuitry 404. Processor circuitry 404 may desirably include readout
circuitry and a signal processor. The read out circuitry electrically
coupled to signal electrode 400 and ground electrode 402, and adapted to
generate a time varying signal proportional to charge (i.e. electrons)
collected by signal electrode 400 as a function of time. Desirably, the
read out circuitry may be monolithically integrated into the ion chamber
as illustrated in FIGS. 3A and 3B.

[0045] The time varying signal generated by the read out circuitry, which
desirably includes a pulse corresponding to each neutron absorbed by
neutron absorbing material 204, is electrically coupled to the signal
processor. The signal processor is adapted to determine path angle 412 of
neutron path 108 relative to normal 410 to top electrode 400 (i.e. the
top surface of the ion chamber) for each absorbed neutron, based on the
rise time of the corresponding pulse in the time varying signal.

[0046] Determining path angle 412 relative to normal 410 to top electrode
400, places neutron path 108 as lying in a cone. This is an improvement
over the lack of directional information available with most neutron
detectors, but it does not provide complete information regarding the
neutron source. For stationary neutron sources, the exemplary directional
neutron detector of FIG. 4 may be adequate. Multiple measurements with
the detector oriented in different directions may allow the precise
direction of the neutron paths to be determined. For moving neutron
sources, however, this method is likely inadequate. To quickly determine
the direction to the neutron source an exemplary detector system
incorporating two or three orthogonally oriented exemplary directional
neutron detectors (as illustrated in FIG. 4). Assuming that there is a
single strong neutron source, the vast majority of the detected neutrons
should be traveling in approximately the same direction. Thus, although
each neutron is detected by only one detector, by identifying the overlap
of the directional cones about their respective normal determined by each
of the individual exemplary directional neutron detectors, the exact
direction to the neutron source may be determined with significant
accuracy by using such an exemplary detector system.

[0047] Although illustrated and described above with reference to certain
specific embodiments, the present invention is nevertheless not intended
to be limited to the details shown. Rather, various modifications may be
made in the details within the scope and range of equivalents of the
claims and without departing from the invention.